Colour correction of digital colour images is required in a number of image processing contexts.
One important environment is in digital imaging. It is known to produce colour sensors by introducing an alternating pattern of colour filters onto the array of individual sensor elements of a device. Alternatively, an image sensor may capture information relating to a plurality of different wavelengths of light at each point of the sensor.
It is, however, difficult to construct colour filters for such sensors which exactly match the spectral characteristics of our eyes or which exactly match the primary colours used in computers to represent or display images. For this reason it is necessary for the captured images to be processed to transform the sensed colours to the desired colour system.
These issues apply to other contexts in which colour correction is required, such as for images generated by imaging systems having three CCD sensors (one for each colour plane) for example, by flatbed colour scanners, or by other imaging systems in which the colour image is formed from sets of separate, registered images. Certain of these issues may apply to other contexts also such as printing, where colour correction is used to map from one colour space to that of the printer such as, for example, from a standard RGB (Red, Green, Blue additive primary) colour space to the RGB space of the printer (prior to the final transformation to the physical CMYK (Cyan, Magenta, Yellow, Black subtractive primary) colour space of the printer).
When processing an image in order to transform it from one colour space to another, it is desirable to avoid mixing noise from a noisy channel, such as the blue colour channel for example, into a less noisy channel, such as the green colour channel for example, whilst still obtaining a satisfactory transformed image.
Co-pending United Kingdom Patent Application Number 0118456.3, incorporated by reference in its entirety herein, discloses a method for the colour correction of images. An image to be processed is split into low and high frequency components and colour correction is applied to the low frequency component only. In this manner, the effect of noise is reduced during the colour correction process as the higher spatial frequency component of the image, which generally carries a larger proportion of the noise in an image, has no colour correction applied to it.
The process of GB0118456.3 is suitable for modest transforms within the same basic colour space such as RGB to RGB, but it does not work particularly well in more extreme situations such as when transforming from complementary colours such as CMY to the primary RGB colours for example.
Both Japanese Patent Application No. 2003-110860 and “Suppression of Noise Amplification During Colour Correction”, Kharitonenko et al., IEEE Transactions on Consumer Electronics, Vol. 48, No. May 2, 2002 (Published), pp. 229-233 describe processes for colour correction of images.
A further enhancement of GB0118456.3, incorporated by reference in its entirety herein, is described in U.S. patent application Ser. No. 10/216,648. Therein, an adjustment may be applied to the high frequency image before recombining it with the colour corrected low frequency image in order to provide additional colour correction around areas of highly chromatic edges.
Despite this improvement and the fact that any high frequency image component processing only occurs around highly chromatic edges, some noise is still introduced into the final transformed image by virtue of the high frequency image component processing. Furthermore, the computation required to robustly identify strong chromatic edges is high, and not entirely suited to use in mobile devices in which processing power is limited.